Abstract
The frequency-domain helicopter-borne electromagnetic (HEM) method is the most suited for resistivity mapping in very large areas. Another advantage of HEM survey is to provide information that allows for 3-D quantitative interpretation of the resistivity distribution. However, in many cases, the information contained in the multi-frequency data is not fully utilized, since conventional approaches to interpreting HEM data are based on the apparent resistivity and 1-D inversion. In this paper, we present the results of applying the 2-D or 3-D inversion method using the finite-difference modeling to two synthetic HEM data sets. The examples indicate that 2-D and 3-D inversions give more accurate representations of the true structures than the analysis of apparent resistivity and the 1-D inversion. The computing time for 3-D inversion is about 20 hours on PC for a survey area including three 1-km flight lines separated by 100 m. This level of efficiency is achieved by taking advantage of the feature unique to the HEM method that the transmitter and receiver coils can be assumed to be superposed, which results in eliminating the pseudo-forward modeling needed to compute the sensitivity matrix. For actual surveys covering much larger areas, it is recommended that the entire survey area should be divided into sub-areas so that 3-D inversion is feasible for each sub-data set on PC.
